Open a panel below two other panels
Grab the three hash marks at the bottom, left-hand corner of the Dope Sheet and drag up to split it. Make sure that the seam between the two Dope Sheets is at the same height as the seam between the two 3D Viewports. Adjust the 3D Viewport's seam to equalize. If the seams are at the same height, you should be able to grab the three hash marks in the top, right-hand corner of the bottom 3D Viewport, drag them to the right, and release to collapse the bottom Dope Sheet.